Waveguides Incorporating Transmissive and Reflective Gratings and Related Methods of Manufacturing

ABSTRACT

Multiplexed reflection and transmission gratings, and methods of their manufacture, are provided that improve uniformity with laser light, that is, reduced banding and other illumination artifacts occurring in waveguides. The mechanism for this can be the multiple reflections between the waveguide reflecting surfaces and the reflection hologram, which promote illumination averaging as beam propagation processes within a waveguide. In some gratings, a beam splitter layer overlapping the multiplexed gratings can be provided for the purposes of reducing banding in a laser-illuminated waveguide. The beam splitter can be provided by one or more dielectric layers. The beamsplitter can have sensitivity to one polarization. The beamsplitter can be sensitive to S-polarization. The beam splitter can be an anti-reflection coating optimized for normal incidence that becomes reflective at high TIR angles when immersed in glass or plastic.

CROSS-REFERENCE TO RELATED APPLICATIONS

The current application is a continuation of U.S. patent applicationSer. No. 16/895,856 entitled “Waveguides Incorporating Transmissive andReflective Gratings and Related Methods of Manufacturing,” filed Jun. 8,2020, which claims the benefit of and priority under 35 U.S.C. § 119(e)to U.S. Provisional Patent Application No. 62/858,928 entitled “SingleGrating Layer Color Holographic Waveguide Displays and Related Methodsof Manufacturing,” filed Jun. 7, 2019, the disclosures of which arehereby incorporated by reference in their entireties.

FIELD OF THE INVENTION

The present disclosure relates to waveguide devices and, moreparticularly, to holographic waveguide displays.

BACKGROUND

Waveguides can be referred to as structures with the capability ofconfining and guiding waves (i.e., restricting the spatial region inwhich waves can propagate). One subclass includes optical waveguides,which are structures that can guide electromagnetic waves, typicallythose in the visible spectrum. Waveguide structures can be designed tocontrol the propagation path of waves using a number of differentmechanisms. For example, planar waveguides can be designed to utilizediffraction gratings to diffract and couple incident light into thewaveguide structure such that the in-coupled light can proceed to travelwithin the planar structure via total internal reflection (TIR).

Fabrication of waveguides can include the use of material systems thatallow for the recording of holographic optical elements within thewaveguides. One class of such material includes polymer dispersed liquidcrystal (PDLC) mixtures, which are mixtures containingphotopolymerizable monomers and liquid crystals. A further subclass ofsuch mixtures includes holographic polymer dispersed liquid crystal(HPDLC) mixtures. Holographic optical elements, such as volume phasegratings, can be recorded in such a liquid mixture by illuminating thematerial with two mutually coherent laser beams. During the recordingprocess, the monomers polymerize, and the mixture undergoes aphotopolymerization-induced phase separation, creating regions denselypopulated by liquid crystal micro-droplets, interspersed with regions ofclear polymer. The alternating liquid crystal-rich and liquidcrystal-depleted regions form the fringe planes of the grating. Theresulting grating, which is commonly referred to as a switchable Bragggrating (SBG), has all the properties normally associated with volume orBragg gratings but with much higher refractive index modulation rangescombined with the ability to electrically tune the grating over acontinuous range of diffraction efficiency (the proportion of incidentlight diffracted into a desired direction). The latter can extend fromnon-diffracting (cleared) to diffracting with close to 100% efficiency.

Waveguide optics, such as those described above, can be considered for arange of display and sensor applications. In many applications,waveguides containing one or more grating layers encoding multipleoptical functions can be realized using various waveguide architecturesand material systems, enabling new innovations in near-eye displays foraugmented reality (AR) and virtual reality (VR), compact head-updisplays (HUDs) and helmet-mounted displays or head-mounted displays(HMDs) for road transport, aviation, and military applications, andsensors for biometric and laser radar (LIDAR) applications.

SUMMARY OF THE INVENTION

Systems and methods for implementing holographic waveguide displaysincorporating transmissive and reflective gratings in accordance withvarious embodiments of the invention are illustrated. One embodimentincludes a waveguide display including a source of light modulated withimage data and a waveguide including at least one transmission grating,at least one reflection grating, wherein the at least one reflection andthe at least one transmission grating at least partially overlap, and atleast one input coupler for coupling light from the source of light intoa TIR path in the waveguide.

In another embodiment, the at least one reflection grating and the atleast one transmission grating are multiplexed in a single gratinglayer.

In a further embodiment, the at least one input coupler is a grating.

In still another embodiment, the at least one input coupler includes aninput transmission grating, the at least one transmission gratingincludes a fold transmission grating and an output transmission grating,and at least one of the input, fold, and output transmission gratings ismultiplexed with the at least one reflection grating.

In a still further embodiment, the at least one input coupler includesan input transmission grating, the at least one transmission gratingincludes first and second fold transmission gratings, the at least onereflection grating overlaps at least one of the input transmissiongrating and the first and second fold transmission gratings, the firstand second fold transmission gratings overlap each other, the first andsecond fold transmission gratings have crossed K-vectors, each of thefold transmission gratings is configured to beam-expand light from theinput grating and couple it towards the other fold transmission grating,which then beam-expand and extract light towards a viewer.

In yet another embodiment, each of the gratings has a grating vectorthat in combination provide a resultant vector with substantially zeromagnitude.

In a yet further embodiment, the light undergoes a dual interactionwithin at least one of the gratings.

In another additional embodiment, the waveguide display further includesa beam splitter layer overlapping the at least one reflection grating.

In a further additional embodiment, the waveguide display furtherincludes an alignment layer overlapping the at least one reflectiongrating.

In another embodiment again, the source of data modulated light is oneof a laser-scanning projector, a microdisplay panel, and/or an emissivedisplay.

In a further embodiment again, the source of light provides at least twodifferent wavelengths.

In still yet another embodiment, at least one of the gratings ischaracterized by a spatial variation of a property that is one ofrefractive index modulation, K-vector, grating vector, grating pitch,and/or birefringence.

In a still yet further embodiment, the gratings are configured toprovide separate optical paths for a property that is one of wavelengthband, angular bandwidth, and/or polarization state.

In still another additional embodiment, the waveguide is curved.

In a still further additional embodiment, the waveguide incorporates aGRIN structure.

In still another embodiment again, the waveguide is plastic.

In a still further embodiment again, at least one of the gratingsincludes a structure that is one of a switchable Bragg grating recordedin a holographic photopolymer a HPDLC material, a switchable Bragggrating recorded in a uniform modulation holographic liquid crystalpolymer material, a Bragg grating recorded in a photopolymer material,and/or a surface relief grating.

A yet another additional embodiment includes a method of fabricating aholographic waveguide, the method including providing at least one lightsource, a layer of holographic recording material, and an at leastpartially reflective surface, forming first and second recording beamsusing the at least one light source, transmitting the first and secondrecording beams into the layer of holographic recording material,transmitting a portion of the first recording beam through the layer ofholographic recording material towards the at least partially reflectivesurface, reflecting the transmitted portion of the first beam off the atleast partially reflective surface back into the layer of holographicrecording material, forming a transmission grating in the layer ofholographic recording material using the first and second recordingbeams, and forming a reflection grating in the layer of holographicrecording material using the reflected portion of the first recordingbeam and the second recording beam.

In a yet further additional embodiment, the method further includesforming a liquid crystal and polymer anchoring structure for supportinga reflection grating.

A yet another embodiment again includes a method of fabricating aholographic waveguide, the method including providing a master grating,a substrate supporting a layer of recording material, a source of light,and an at least partially reflective surface disposed opposite to themaster grating with respect to the layer of recording material,illuminating the master grating with light from the source of light toform a diffracted beam and a zero-order beam, reflecting the diffractedbeam from the at least partially reflective surface, forming atransmission grating from the zero-order beam and the diffracted beam,and forming a reflection grating from the zero-order beam and thereflected diffracted beam.

Additional embodiments and features are set forth in part in thedescription that follows, and in part will become apparent to thoseskilled in the art upon examination of the specification or may belearned by the practice of the invention. A further understanding of thenature and advantages of the present invention may be realized byreference to the remaining portions of the specification and thedrawings, which forms a part of this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The description will be more fully understood with reference to thefollowing figures and data graphs, which are presented as exemplaryembodiments of the invention and should not be construed as a completerecitation of the scope of the invention.

FIG. 1 conceptually illustrates a waveguide display implementingtransmission and reflection gratings in accordance with an embodiment ofthe invention.

FIG. 2 conceptually illustrates a waveguide display having multiplexedtransmission and reflection gratings in accordance with an embodiment ofthe invention.

FIG. 3 conceptually illustrates multiplexed transmission and reflectiongratings in accordance with an embodiment of the invention.

FIG. 4 conceptually illustrates a system for recording a multiplexedtransmission-reflection grating in accordance with an embodiment of theinvention.

FIGS. 5 and 6 conceptually illustrate the formation of multiplexedtransmission and reflection gratings in accordance with variousembodiments of the invention.

FIGS. 7-9 show flow diagrams conceptually illustrating different methodsof forming transmission and reflection gratings in accordance withvarious embodiments of the invention.

FIG. 10 conceptually illustrates a waveguide display architectureimplementing multiplexed transmission and reflection fold gratings inaccordance with an embodiment of the invention.

FIG. 11 conceptually illustrates a waveguide architecture implementingmultiplexed transmission and reflection output gratings in accordancewith an embodiment of the invention.

FIG. 12 conceptually illustrates a waveguide architecture implementingmultiplexed transmission and reflection input gratings in accordancewith an embodiment of the invention.

FIG. 13 conceptually illustrates a waveguide architecture implementingtwo separate input gratings in accordance with an embodiment of theinvention.

FIG. 14 conceptually illustrates a waveguide architecture implementingcrossed fold gratings in accordance with an embodiment of the invention.

FIG. 15 conceptually illustrates a waveguide architecture implementingcrossed fold gratings in which the input coupler multiplexestransmission and reflection gratings in accordance with an embodiment ofthe invention.

FIG. 16 conceptually illustrates a waveguide architecture implementingcrossed fold gratings in which two separate input gratings are providedin accordance with an embodiment of the invention.

FIG. 17 conceptually illustrates a profile view of a waveguidearchitecture in which overlapping transmission and reflection gratingsare provided in accordance with an embodiment of the invention.

FIG. 18 shows a chart illustrating the diffraction efficiency versusincidence angle of a reflection grating and the diffraction efficiencyangular bandwidth of a transmission grating in accordance with anembodiment of the invention.

FIG. 19 conceptually illustrates a profile view of a waveguidearchitecture in which a transmission grating and four reflectiongratings are provided in accordance with an embodiment of the invention.

FIG. 20 shows a chart illustrating the diffraction efficiency versusincidence angle of a reflection grating, the effective angular bandwidthresulting from the two reflection gratings, and the diffractionefficiency angular bandwidth of the transmission grating in accordancewith an embodiment of the invention.

DETAILED DESCRIPTION

For the purposes of describing embodiments, some well-known features ofoptical technology known to those skilled in the art of optical designand visual displays have been omitted or simplified in order to notobscure the basic principles of the invention. Unless otherwise stated,the term “on-axis” in relation to a ray or a beam direction refers topropagation parallel to an axis normal to the surfaces of the opticalcomponents described in relation to the invention. In the followingdescription the terms light, ray, beam, and direction may be usedinterchangeably and in association with each other to indicate thedirection of propagation of electromagnetic radiation along rectilineartrajectories. The term light and illumination may be used in relation tothe visible and infrared bands of the electromagnetic spectrum. Parts ofthe following description will be presented using terminology commonlyemployed by those skilled in the art of optical design. As used herein,the term grating may encompass a grating having a set of gratings insome embodiments. For illustrative purposes, it is to be understood thatthe drawings are not drawn to scale unless stated otherwise.

Waveguide technology can enable low cost, efficient, and versatilediffractive optical solutions for many different applications. In manyembodiments, a waveguide display supporting at least one transmissiongrating and at least one reflection grating is implemented. Thetransmission and reflection gratings can be implemented across differentgrating layers or within a single grating layer. In some embodiments,the transmission and reflection gratings are multiplexed. A multiplexedtransmission and reflection grating can be configured for the specificpurpose of supporting transmission gratings at angles that otherwisecould not be supported in typical Bragg gratings. In severalembodiments, such structures can be used to make high efficiencyreflection input gratings for use in waveguides.

In many embodiments, multiplexed reflection and transmission gratingscan provide improved uniformity with laser light, that is, reducedbanding and other illumination artifacts occurring in waveguides. Themechanism for this can be the multiple reflections between the waveguidereflecting surfaces and the reflection hologram, which promoteillumination averaging as beam propagation processes within a waveguide.In some embodiments, a beam splitter layer overlapping the multiplexedgratings can be provided for the purposes of reducing banding in alaser-illuminated waveguide. The beam splitter can be provided by one ormore dielectric layers. In several embodiments, the beamsplitter canhave sensitivity to one polarization. In further embodiments, thebeamsplitter can be sensitive to S-polarization. In a number ofembodiments, the beam splitter can be an anti-reflection coatingoptimized for normal incidence that becomes reflective at high TIRangles when immersed in glass or plastic.

Various systems and methods can be implemented to fabricate waveguidesincorporating transmissive and reflective gratings. In many embodiments,a system for fabricating such gratings can include at least one sourceof light, a master grating providing a zero-order beam and at least onediffracted order beam from the light, a substrate supporting a layer ofholographic recording material (such as but not limited to HPDLCmaterials) overlapping the master, and an at least partially reflectivesurface overlapping the holographic recording material layer. During therecording operation, the diffracted beam can be reflected by the atleast partially reflective surface. Through a combination ofinterference from the zero-order beam, the diffracted beam, and thereflected beam, both transmission and reflection gratings can berecorded. In many embodiments, the transmission and reflection gratingsare multiplexed. In some embodiments, the system includes an HPDLCmixture that includes a weak dielectric material that enables efficientmultiplexing of reflection and transmission gratings without generatingunwanted reflections (and hence spurious gratings). In severalembodiments, overlaid alignment layers may be used to fine tune HPDLCmultiplexed reflection and transmission grating formation. For example,in some embodiments, selective alignment of HPDLC gratings can be usedto balance the refractive index modulations and or the polarizationresponse of the multiplexed transmission and reflection gratings. In anumber of embodiments, alignment layers may be used to promoteS-polarization sensitivity in the reflection grating. In a typicalwaveguide implementation, the average extraordinary axis of the LC richfringes (which in typical HPDLC gratings will be orthogonal to the Braggfringe plane) will be normal to the waveguide reflecting surfaces. Thisorientation can be advantageous for providing strong interaction withlight propagating through a fold grating at typical waveguide totalinternal reflection angles.

Waveguide embodiments implementing transmission and reflection gratingscan be utilized and configured for a variety of applications. Forexample, in some applications, it is desirable for the waveguide to becompact and wide angle with a generous eyebox while also providing fullcolor. Previous solutions to color imaging have include stacking two ormore monochrome waveguides, where each waveguide supports a gratinglayer with gratings configured to operate in a single color. In manycases, each waveguide is further configured for inputting imagemodulated light, expanding the light in two dimensions, and extractingit from the waveguide towards an eye box. However, such multi-waveguidestacking solutions suffer from the tight tolerances required to alignthe overlapping gratings in the waveguide stack, which can result in lowmanufacturing yield. Two-layer solutions in which one layer propagatesred light and the second layer propagate light in the green-blue bandhave been attempted but still present alignment problems inmanufacturing. As such, many embodiments of the invention are directedtowards methods and architectures for implementing wide-angle, singlegrating layer color waveguide displays. Waveguide and gratingarchitectures, holographic recording materials, and waveguideembodiments incorporating transmission and reflection gratings arediscussed in the sections below in further detail.

Optical Waveguide and Grating Structures

Optical structures recorded in waveguides can include many differenttypes of optical elements, such as but not limited to diffractiongratings. Gratings can be implemented to perform various opticalfunctions, including but not limited to coupling light, directing light,and preventing the transmission of light. In many embodiments, thegratings are surface relief gratings that reside on the outer surface ofthe waveguide. In other embodiments, the grating implemented is a Bragggrating (also referred to as a volume grating), which are structureshaving a periodic refractive index modulation. Bragg gratings can befabricated using a variety of different methods. One process includesinterferential exposure of holographic photopolymer materials to formperiodic structures. Bragg gratings can have high efficiency with littlelight being diffracted into higher orders. The relative amount of lightin the diffracted and zero order can be varied by controlling therefractive index modulation of the grating, a property that can be usedto make lossy waveguide gratings for extracting light over a largepupil. A grating can be characterized by a grating vector defining theorientation of the grating fringes in the plane of the waveguide. Agrating can also be characterized by a K-vector in 3D space, which inthe case of a Bragg grating is defined as the vector normal to the Braggfringes. The K-vector vector can determine the optical efficiency for agiven range of input and diffracted angles.

One class of Bragg gratings used in holographic waveguide devices is theSwitchable Bragg Grating (SBG). SBGs can be fabricated by first placinga thin film of a mixture of photopolymerizable monomers and liquidcrystal material between substrates. The substrates can be made ofvarious types of materials, such glass and plastics. In many cases, thesubstrates are in a parallel configuration. In other embodiments, thesubstrates form a wedge shape. One or both substrates can supportelectrodes, typically transparent tin oxide films, for applying anelectric field across the film. The grating structure in an SBG can berecorded in the liquid material (often referred to as the syrup) throughphotopolymerization-induced phase separation using interferentialexposure with a spatially periodic intensity modulation. Factors such asbut not limited to control of the irradiation intensity, componentvolume fractions of the materials in the mixture, and exposuretemperature can determine the resulting grating morphology andperformance. As can readily be appreciated, a wide variety of materialsand mixtures can be used depending on the specific requirements of agiven application. In many embodiments, HPDLC material is used. Duringthe recording process, the monomers polymerize, and the mixtureundergoes a phase separation. The LC molecules aggregate to formdiscrete or coalesced droplets that are periodically distributed inpolymer networks on the scale of optical wavelengths. The alternatingliquid crystal-rich and liquid crystal-depleted regions form the fringeplanes of the grating, which can produce Bragg diffraction with a strongoptical polarization resulting from the orientation ordering of the LCmolecules in the droplets.

The resulting volume phase grating can exhibit very high diffractionefficiency, which can be controlled by the magnitude of the electricfield applied across the film. When an electric field is applied to thegrating via transparent electrodes, the natural orientation of the LCdroplets can change, causing the refractive index modulation of thefringes to lower and the hologram diffraction efficiency to drop to verylow levels. Typically, the electrodes are configured such that theapplied electric field will be perpendicular to the substrates. In anumber of embodiments, the electrodes are fabricated from indium tinoxide (ITO). In the OFF state with no electric field applied, theextraordinary axis of the liquid crystals generally aligns normal to thefringes. The grating thus exhibits high refractive index modulation andhigh diffraction efficiency for P-polarized light. When an electricfield is applied to the HPDLC, the grating switches to the ON statewherein the extraordinary axes of the liquid crystal molecules alignparallel to the applied field and hence perpendicular to the substrate.In the ON state, the grating exhibits lower refractive index modulationand lower diffraction efficiency for both S- and P-polarized light.Thus, the grating region no longer diffracts light. Each grating regioncan be divided into a multiplicity of grating elements such as forexample a pixel matrix according to the function of the HPDLC device.Typically, the electrode on one substrate surface is uniform andcontinuous, while electrodes on the opposing substrate surface arepatterned in accordance to the multiplicity of selectively switchablegrating elements.

Typically, the SBG elements are switched clear in 30 μs with a longerrelaxation time to switch ON. The diffraction efficiency of the devicecan be adjusted, by means of the applied voltage, over a continuousrange. In many cases, the device exhibits near 100% efficiency with novoltage applied and essentially zero efficiency with a sufficiently highvoltage applied. In certain types of HPDLC devices, magnetic fields canbe used to control the LC orientation. In some HPDLC applications, phaseseparation of the LC material from the polymer can be accomplished tosuch a degree that no discernible droplet structure results. An SBG canalso be used as a passive grating. In this mode, its chief benefit is auniquely high refractive index modulation. SBGs can be used to providetransmission or reflection gratings for free space applications. SBGscan be implemented as waveguide devices in which the HPDLC forms eitherthe waveguide core or an evanescently coupled layer in proximity to thewaveguide. The substrates used to form the HPDLC cell provide a totalinternal reflection (TIR) light guiding structure. Light can be coupledout of the SBG when the switchable grating diffracts the light at anangle beyond the TIR condition.

In some embodiments, LC can be extracted or evacuated from the SBG toprovide an evacuated Bragg grating (EBG). EBGs can be characterized as asurface relief grating (SRG) that has properties very similar to a Bragggrating due to the depth of the SRG structure (which is much greaterthan that practically achievable using surface etching and otherconventional processes commonly used to fabricate SRGs). The LC can beextracted using a variety of different methods, including but notlimited to flushing with isopropyl alcohol and solvents. In manyembodiments, one of the transparent substrates of the SBG is removed,and the LC is extracted. In further embodiments, the removed substrateis replaced. The SRG can be at least partially backfilled with amaterial of higher or lower refractive index. Such gratings offer scopefor tailoring the efficiency, angular/spectral response, polarization,and other properties to suit various waveguide applications.

Waveguides in accordance with various embodiments of the invention caninclude various grating configurations designed for specific purposesand functions. In many embodiments, the waveguide is designed toimplement a grating configuration capable of preserving eyebox sizewhile reducing lens size by effectively expanding the exit pupil of acollimating optical system. The exit pupil can be defined as a virtualaperture where only the light rays which pass though this virtualaperture can enter the eyes of a user. In some embodiments, thewaveguide includes an input grating optically coupled to a light source,a fold grating for providing a first direction beam expansion, and anoutput grating for providing beam expansion in a second direction, whichis typically orthogonal to the first direction, and beam extractiontowards the eyebox. As can readily be appreciated, the gratingconfiguration implemented waveguide architectures can depend on thespecific requirements of a given application. In many embodiments, thegratings used in any of the embodiments can have grating vectors matchedto provide a resultant vector with substantially zero magnitude. In someembodiments, the grating configuration includes multiple fold gratings.In several embodiments, the grating configuration includes an inputgrating and a second grating for performing beam expansion and beamextraction simultaneously. The second grating can include gratings ofdifferent prescriptions, for propagating different portions of thefield-of-view, arranged in separate overlapping grating layers ormultiplexed in a single grating layer. In a number of embodiments, twograting layers are disposed on either side of a single substrate layer.Furthermore, various types of gratings and waveguide architectures canalso be utilized.

In several embodiments, the gratings within each layer are designed tohave different spectral and/or angular responses. For example, in manyembodiments, different gratings across different grating layers areoverlapped, or multiplexed, to provide an increase in spectralbandwidth. In some embodiments, a full color waveguide is implementedusing three grating layers, each designed to operate in a differentspectral band (red, green, and blue). In other embodiments, a full colorwaveguide is implemented using two grating layers, a red-green gratinglayer and a green-blue grating layer. As can readily be appreciated,such techniques can be implemented similarly for increasing angularbandwidth operation of the waveguide. In addition to the multiplexing ofgratings across different grating layers, multiple gratings can bemultiplexed within a single grating layer—i.e., multiple gratings can besuperimposed within the same volume. In several embodiments, thewaveguide includes at least one grating layer having two or more gratingprescriptions multiplexed in the same volume. In further embodiments,the waveguide includes two grating layers, each layer having two gratingprescriptions multiplexed in the same volume. Multiplexing two or moregrating prescriptions within the same volume can be achieved usingvarious fabrication techniques. In a number of embodiments, amultiplexed master grating is utilized with an exposure configuration toform a multiplexed grating. In many embodiments, a multiplexed gratingis fabricated by sequentially exposing an optical recording materiallayer with two or more configurations of exposure light, where eachconfiguration is designed to form a grating prescription. In someembodiments, a multiplexed grating is fabricated by exposing an opticalrecording material layer by alternating between or among two or moreconfigurations of exposure light, where each configuration is designedto form a grating prescription. As can readily be appreciated, varioustechniques, including those well known in the art, can be used asappropriate to fabricate multiplexed gratings.

In some embodiments, the light propagating within a waveguide inaccordance with an embodiment of the invention can undergo a dualinteraction within at least one of the gratings (i.e., the grating isdesigned to have high diffraction efficiency, or diffraction efficiencypeaks, for two different incidence angles). In many embodiments, thewaveguide can incorporate at least one of: angle multiplexed gratings,color multiplexed gratings, fold gratings, dual interaction gratings,rolled K-vector gratings, crossed fold gratings, tessellated gratings,chirped gratings, gratings with spatially varying refractive indexmodulation, gratings having spatially varying grating thickness,gratings having spatially varying average refractive index, gratingswith spatially varying refractive index modulation tensors, gratingshaving spatially varying average refractive index tensors, and gratingshaving spatially varying birefringence properties. In some embodiments,the waveguide can incorporate at least one of: a half wave plate, aquarter wave plate, an anti-reflection coating, a beam splitting layer,an alignment layer, a photochromic back layer for glare reduction, andlouvre films for glare reduction. In several embodiments, the waveguidecan support gratings providing separate optical paths for differentpolarizations. In various embodiments, the waveguide can supportgratings providing separate optical paths for different spectral and/orangular bandwidths. In a number of embodiments, the gratings can beHPDLC gratings, switching gratings recorded in HPDLC (such switchableBragg Gratings), Bragg gratings recorded in holographic photopolymer, orsurface relief gratings. In many embodiments, the waveguide operates ina monochrome band. In some embodiments, the waveguide operates in thegreen band. In several embodiments, waveguide layers operating indifferent spectral bands such as red, green, and blue (RGB) can bestacked to provide a three-layer waveguiding structure. In furtherembodiments, the layers are stacked with air gaps between the waveguidelayers. In various embodiments, the waveguide layers operate in broaderbands such as blue-green and green-red to provide two-waveguide layersolutions. In other embodiments, the gratings are color multiplexed toreduce the number of grating layers. Various types of gratings can beimplemented. In some embodiments, at least one grating in each layer isa switchable grating. In many embodiments, the waveguide can be curved.In several embodiments, the waveguide can incorporate a gradient index(GRIN) structure. In a number of embodiments, the waveguide can befabricated using plastic substrates.

Waveguides incorporating optical structures such as those discussedabove can be implemented in a variety of different applications,including but not limited to waveguide displays. In various embodiments,the waveguide display is implemented with an eyebox of greater than 10mm with an eye relief greater than 25 mm. In some embodiments, thewaveguide display includes a waveguide with a thickness between 2.0-5.0mm. In many embodiments, the waveguide display can provide an imagefield-of-view of at least 50° diagonal. In further embodiments, thewaveguide display can provide an image field-of-view of at least 70°diagonal. The waveguide display can employ many different types ofpicture generation units (PGUs). In several embodiments, the PGU can bea reflective or transmissive spatial light modulator such as a liquidcrystal on Silicon (LCoS) panel or a micro electromechanical system(MEMS) panel. In a number of embodiments, the PGU can be an emissivedevice such as an organic light emitting diode (OLED) panel. In someembodiments, an OLED display can have a luminance greater than 4000 nitsand a resolution of 4 k×4 k pixels. In several embodiments, thewaveguide can have an optical efficiency greater than 10% such that agreater than 400 nit image luminance can be provided using an OLEDdisplay of luminance 4000 nits. Waveguides implementing P-diffractinggratings (i.e., gratings with high efficiency for P-polarized light)typically have a waveguide efficiency of 5%-6.2%. Since P-diffracting orS-diffracting gratings can waste half of the light from an unpolarizedsource such as an OLED panel, many embodiments are directed towardswaveguides capable of providing both S-diffracting and P-diffractinggratings to allow for an increase in the efficiency of the waveguide byup to a factor of two. In some embodiments, the S-diffracting andP-diffracting gratings are implemented in separate overlapping gratinglayers. Alternatively, a single grating can, under certain conditions,provide high efficiency for both p-polarized and s-polarized light. Inseveral embodiments, the waveguide includes Bragg-like gratings producedby extracting LC from HPDLC gratings, such as those described above, toenable high S and P diffraction efficiency over certain wavelength andangle ranges for suitably chosen values of grating thickness (typically,in the range 2-5 μm).

Optical Recording Material Systems

HPDLC mixtures generally include LC, monomers, photoinitiator dyes, andcoinitiators. The mixture (often referred to as syrup) frequently alsoincludes a surfactant. For the purposes of describing the invention, asurfactant is defined as any chemical agent that lowers the surfacetension of the total liquid mixture. The use of surfactants in PDLCmixtures is known and dates back to the earliest investigations ofPDLCs. For example, a paper by R. L Sutherland et al., SPIE Vol. 2689,158-169, 1996, the disclosure of which is incorporated herein byreference, describes a PDLC mixture including a monomer, photoinitiator,coinitiator, chain extender, and LCs to which a surfactant can be added.Surfactants are also mentioned in a paper by Natarajan et al, Journal ofNonlinear Optical Physics and Materials, Vol. 5 No. I 89-98, 1996, thedisclosure of which is incorporated herein by reference. Furthermore,U.S. Pat. No. 7,018,563 by Sutherland; et al., discussespolymer-dispersed liquid crystal material for forming apolymer-dispersed liquid crystal optical element having: at least oneacrylic acid monomer; at least one type of liquid crystal material; aphotoinitiator dye; a coinitiator; and a surfactant. The disclosure ofU.S. Pat. No. 7,018,563 is hereby incorporated by reference in itsentirety.

The patent and scientific literature contains many examples of materialsystems and processes that can be used to fabricate SBGs, includinginvestigations into formulating such material systems for achieving highdiffraction efficiency, fast response time, low drive voltage, and soforth. U.S. Pat. No. 5,942,157 by Sutherland, and U.S. Pat. No.5,751,452 by Tanaka et al. both describe monomer and liquid crystalmaterial combinations suitable for fabricating SBG devices. Examples ofrecipes can also be found in papers dating back to the early 1990s. Manyof these materials use acrylate monomers, including:

-   -   R. L. Sutherland et al., Chem. Mater. 5, 1533 (1993), the        disclosure of which is incorporated herein by reference,        describes the use of acrylate polymers and surfactants.        Specifically, the recipe includes a crosslinking multifunctional        acrylate monomer; a chain extender N-vinyl pyrrolidinone, LC E7,        photoinitiator rose Bengal, and coinitiator N-phenyl glycine.        Surfactant octanoic acid was added in certain variants.    -   Fontecchio et al., SID 00 Digest 774-776, 2000, the disclosure        of which is incorporated herein by reference, describes a UV        curable HPDLC for reflective display applications including a        multi-functional acrylate monomer, LC, a photoinitiator, a        coinitiators, and a chain terminator.    -   Y. H. Cho, et al., Polymer International, 48, 1085-1090, 1999,        the disclosure of which is incorporated herein by reference,        discloses HPDLC recipes including acrylates.    -   Karasawa et al., Japanese Journal of Applied Physics, Vol. 36,        6388-6392, 1997, the disclosure of which is incorporated herein        by reference, describes acrylates of various functional orders.    -   T. J. Bunning et al., Polymer Science: Part B: Polymer Physics,        Vol. 35, 2825-2833, 1997, the disclosure of which is        incorporated herein by reference, also describes multifunctional        acrylate monomers.    -   G. S. Iannacchione et al., Europhysics Letters Vol. 36 (6).        425-430, 1996, the disclosure of which is incorporated herein by        reference, describes a PDLC mixture including a penta-acrylate        monomer, LC, chain extender, coinitiators, and photoinitiator.

Acrylates offer the benefits of fast kinetics, good mixing with othermaterials, and compatibility with film forming processes. Sinceacrylates are cross-linked, they tend to be mechanically robust andflexible. For example, urethane acrylates of functionality 2 (di) and 3(tri) have been used extensively for HPDLC technology. Higherfunctionality materials such as penta and hex functional stems have alsobeen used.

Waveguides Incorporating Reflection and Transmission Gratings

Referring generally to the drawings, systems and methods relating todisplays or sensors implementing full color in a single grating layer inaccordance with various embodiments of the invention are illustrated. Inmany embodiments, a waveguide display according to the principles of theinvention includes at least one waveguide substrate, a source of lightmodulated with image data, at least one input coupler for coupling thelight into TIR in waveguide, at least one transmission grating, and atleast one reflection grating, where the reflection and the transmissiongrating at least partially overlap. FIG. 1 conceptually illustrates awaveguide display implementing transmission and reflection gratings inaccordance with an embodiment of the invention. As shown, the display100 includes a waveguide 101 supporting an input grating 102 providingthe functions of an input coupler, a transmission grating 103, and areflection grating 104. In the illustrative embodiment, the transmissiongrating 103 and the reflection grating 104 at least partially overlap.In some embodiments, the reflection grating and the transmissiongratings can be multiplexed in a single grating layer. FIG. 2conceptually illustrates such a waveguide. In FIG. 2, the display 200shown includes a waveguide 201 supporting an input grating 202 andmultiplexed transmission 203 and reflection gratings 204. Although FIGS.1 and 2 illustrate specific waveguide structures, various configurationsand modifications can be implemented in accordance with variousembodiments of the invention. For example, in several embodiments, aprism is utilized instead of a grating as the input coupler. Asdiscussed above, such waveguide displays can include at least one lightsource. In some embodiments, the light source provides image modulatedlight. In a number of embodiments, the source of data modulated lightcan include at least one of: a laser-scanning projector, a microdisplaypanel, and/or an emissive display. In several embodiments, the source ofdata modulated light can provide at least two different wavelengths oflight.

FIG. 3 conceptually illustrates multiplexed transmission and reflectiongratings in accordance with an embodiment of the invention. As shown,the multiplexed grating 300 includes a transmission grating havingfringes 301 separated by regions containing reflection gratingscharacterized by low refractive index 302 and high refractive indexfringes 303. In many embodiments, the fringes 301 of the transmissiongrating can have an index greater than the average index of thereflection grating. In other embodiments, the fringes 301 of thetransmission grating can have an index less than the average index ofthe reflection grating. In some embodiments, the index values areselected for producing various index contrasts between the transmissionand reflection grating fringes. In the embodiment illustrated in FIG. 3,the reflection and transmission fringes are unslanted with transmissiongrating K-vectors 304 (labelled by symbol K_(T)) and reflection gratingK-vectors 305 (labelled by symbol K_(R)) disposed orthogonally to eachother. As can readily be appreciated, other embodiments can includearchitectures where one or both of the transmission and reflectiongratings have slanted fringes.

The gratings as described above and throughout this disclosure caninclude various grating structures, including but not limited to volumegratings and surface relief gratings. In many embodiments, at least oneof the gratings is recorded in a holographic photopolymer, an HPDLCmaterial, or a uniform modulation holographic liquid crystal polymermaterial. Reflection gratings recorded in HPDLC materials can sufferfrom the problem that the resulting Bragg fringes tend to be very longand exhibit poor surface anchoring. In some cases, this can lead todelamination of the grating structure. In embodiments using HPDLCs (suchas the one in FIG. 4), a liquid crystal and polymer anchoring structure(306, 307) that allows a reflection grating to be supported by thefringes of the transmission can be provided. In many embodiments, theanchoring strength can be controlled by selecting LCs and monomers typesand by mixing the LC and monomers in concentrations that promote robustlocal anchoring between LC and polymer. In some embodiments, stronganchoring can be achieved by additives. The term “scaffolding” can beused to describe the use of one grating to support the other'sformation. Relevant data and teachings on the chemistry and processesfor promoting efficient anchoring can be found in the literature ofHPDLC material systems.

Multiplexed gratings, such as the one shown in FIG. 2, can be fabricatedin many different ways. As can readily be appreciated, the specific typeof multiplexed gratings to be formed can dictate the method utilized.FIG. 4 conceptually illustrates a system for recording a multiplexedtransmission-reflection grating in accordance with an embodiment of theinvention. As shown, the recording apparatus 400 includes the following:a master grating substrate 401, a master grating 402, a master coverglass 403, a grating bottom substrate 404, a grating layer 405 ofholographic recording material, a grating top substrate 406, a partiallyreflective layer 407 formed on a lower face of a substrate 408, and afilter glass substrate 409. As can readily be appreciated, each layercan be implemented with various types of materials having variousthicknesses. For example, the master grating 402 can be an amplitudegrating or a volume grating. The master cover glass 403 can beimplemented with ˜1.1 mm thickness optical glass. In other embodiments,different glass thicknesses and materials can be used. The gratingsubstrate layers 404, 406 can be Corning Iris™ glass, which typicallyranges from ˜0.2 mm to ˜-1.8 mm in thickness. In many embodiments, thegrating layer 405 is on the order of micrometers, which can range from˜1 μm to ˜5 μm in thickness. However, other grating layer thicknessescan be used as appropriate depending on the application. In someembodiments, the grating layer 405 is configured with a specificthickness to achieve specific grating angular bandwidth andefficiencies. The recording material of the grating layer 405 can beused to record gratings of any type, including slanted and non-slantedgratings. Such gratings can also be configured for providing variousoptical functions, including but not limited to coupling light into thewaveguide, providing beam expansion, and extracting light from thewaveguide. In a number of embodiments, the partially reflective coating407 can be an antireflection coating that provides appreciablereflection at high incidence angles when immersed in glass. In someembodiments, the partially reflective coating 407 can be provided by oneor more dielectric layers or by a stack comprising dielectric/metallayers. The partially reflective coating glass substrate 408 can beCorning® EAGLE XG® Slim Glass, and the filter glass substrate 409 can beSchott R60 blocker glass.

During the recording process, the master grating 402 can be illuminatedto form zero-order and diffracted light. At least a portion of thezero-order light and at least a portion of the diffracted light cantogether form an interference pattern within the holographic recordingmaterial layer 405 to form a transmission grating. At least a portion ofthe zero-order light can be reflected from the partially reflectingcoating 407 and interferes with at least a portion of the diffractedlight within the holographic recording material 405 to form a reflectiongrating. The reflection and transmission gratings can be formed in asingle multiplexed layer. As can readily be appreciated, in someembodiments, multiple grating layers are utilized to form overlappingtransmission and reflection gratings

FIG. 5 conceptually illustrates the formation of multiplexedtransmission and reflection gratings in accordance with an embodiment ofthe invention. The recording system 500 is similar to the system of FIG.4. Similar to FIG. 4, the recording system 500 can also include a filterglass substrate. As shown, the master grating 501 is illuminated byincident collimated light represented by rays 502-504. The mastergrating 501 produces zero order light rays 505-507 and diffracted lightray 508. The zero order light rays 505-507 passes through a gratinglayer 509. A portion of the diffracted light 508 is reflected (510) offthe upper surface of the top substrate 511 and re-interacts with thegrating layer 509. In the illustrative embodiment, the system 500includes a partially reflecting layer 512. In other embodiments, thislayer is excluded. Referring back to FIG. 5, the reflection from thepartially reflecting layer 512 is substantially weaker than that fromthe upper surface of the top substrate 511. As to the formation of thegratings, this configuration and arrangement of illumination and lightpaths allow for he zero-order light and diffracted light can interfereto form a transmission grating. For example, zero order rays 506 caninterfere with the diffracted rays 508 to form a transmission grating inone portion 513 of the grating layer. Concurrently, the reflecteddiffracted rays 510 can interfere with zero order rays 507 to form areflected grating in another portion 514 of the grating layer. As canreadily be appreciated, the schematic shown in FIG. 5 does notillustrate every single light ray and interference interaction in thesystem. From consideration of the ray paths illustrated in FIG. 5, itshould be apparent that transmission and reflection gratings can bemultiplexed at each point across the grating layer. In some embodiments,the master grating 501 can be illuminated (sequentially orsimultaneously) by more than one incident collimated beam to enable therecording of multiple sets of multiplexed reflection and transmissiongratings. In several embodiments, the master 501 can be illuminated withbeams having different incident angles at different points over theaperture of the master. In a number of embodiments, the master 501 canbe illuminated by a scanned collimated beam. In some embodiments, themaster 501 can be illuminated by a collimated beam that is directed atthe master in a stepwise fashion across the aperture of the master.

FIG. 6 conceptually illustrates another configuration for forming amultiplexed transmission-reflection grating in accordance with anembodiment of the invention. Again, the recording system 600 is similarto the system shown in FIG. 4. Similar to FIG. 4, the recording system600 can also include a filter glass substrate. In contrast to theembodiment of FIG. 5, diffracted light 601 in this case undergoes areflection at the partially reflecting layer 602, the reflection at theupper surface of the top substrate 603 being substantially smaller. Asshown, the master grating 604 is illuminated by incident collimatedlight represented by the rays 605-607. The master grating 604 produceszero order 608-610 light, which passes through the grating layer 611,and the diffracted light 601. A portion of the diffracted light 601 isreflected 612 off the partially reflecting layer 602 and re-interactswith the grating layer 611. The zero-order light and diffracted lightcan interfere to form a transmission grating and/or a reflectiongrating. For example, zero order light rays 609 interfere with thediffracted rays 601 to form a transmission grating in one portion 613 ofthe grating layer, while reflected diffracted rays 612 interfere withzero order rays 610 to form a reflection grating in another portion 614of the grating layer 611.

Although FIGS. 5 and 6 illustrate specific methods and configurations ofrecording transmission and reflection gratings, various other processescan be implemented as appropriate depending on the specific requirementsof a given application. In many embodiments, multiple grating layers areutilized, and the transmission and reflection gratings are notmultiplexed. The multiple grating layers can be adjacent. In someembodiments, two grating layers are disposed on either side of a singlesubstrate layer. In addition to different recording and exposure setup,various different processes can also be implemented.

FIG. 7 shows a flow diagram conceptually illustrating a method offabricating transmission and reflection gratings in accordance with anembodiment of the invention. As shown, the method 700 includes providing(701) at least one light source, a layer of holographic recordingmaterial, and an at least partially reflective surface. As can readilybe appreciated, any of a variety of holographic recording materialincluding but not limited to HPDLC materials and various photopolymerscan be utilized as appropriate. In a number of embodiments, thereflective surface is a fully reflective surface. First and secondrecording beams can be formed (702) using the at least one light source.In many embodiments, a single light source is utilized to the form thefirst and second recording beams. For example, the two beams can beformed by directing a single beam from the light source towards adiffraction grating. In other embodiments, two light sources areutilized to respectively form the first and second recording beams. Thefirst and second recording beams can be transmitted (703) into the layerof holographic recording material. A portion of the first recording beamcan be transmitted (704) through the layer of holographic recordingmaterial towards the at least partially reflective surface. The portionof transmitted first recording beam can be reflected (705) off the atleast partially reflective surface back into the layer of holographicrecording material. A transmission grating can be formed (706) in thelayer of holographic recording material using the first and secondrecording beams. A reflection grating can be formed (707) in the layerof holographic recording material using the reflected first recordingbeam and the second recording beam. In some embodiments, the at leastpartially reflective surface can form part of the finished waveguidecomponent. In several embodiments, the at least partially reflectivesurface (and its supporting substrate) is only present during exposure.

FIG. 8 shows a flow diagram conceptually illustrating a second method offabricating transmission and reflection gratings in accordance with anembodiment of the invention. As shown, the method 800 includes providing(801) at least one light source, a layer of holographic recordingmaterial that includes a monomer and a liquid crystal, and an at leastpartially reflective surface. First and second recording beams can beformed (802) using the at least one light source. The first and secondrecording beams can be transmitted (803) into the layer of holographicrecording material. A portion of the first recording beam can betransmitted (804) through the layer of holographic recording materialtowards the reflective surface. A portion of the transmitted firstrecording beam can be reflected (805) off the reflective surface backinto the layer of holographic recording material. A transmission gratingcan be formed (806) in the layer of holographic recording material usingthe first and second beams. A liquid crystal to polymer anchoringstructure can be formed (807) in the transmission grating for supportinga reflection grating. A reflection grating can be formed layer ofholographic recording material (808) using the reflected first beam andthe second beam.

FIG. 9 shows a flow diagram conceptually illustrating a third method offabricating transmission and reflection gratings in accordance with anembodiment of the invention. As shown, the method 900 includes providing(901) a master grating, a substrate supporting a layer of recordingmaterial, a source of light, and an at least partially reflectivesurface disposed opposite to the master grating with respect to thelayer of recording material (i.e., the layer of recording material isbetween the at least partially reflective surface and the mastergrating). The master grating can be illuminated (902) with light fromthe source of light. A diffracted beam and a zero-order beam can beformed (903) from the illumination of the master grating. At least aportion of the diffracted beam can be reflected (904) from the at leastpartially reflective surface. The zero-order and diffracted beams can beinterfered (905) to form a transmission grating in the layer ofrecording material. The zero-order and reflected beams can be interfered(906) to form a reflection grating in the layer of recording material.As can readily be appreciated, the systems and components implementingthe processes described in FIGS. 8 and 9 can be implemented similarly tothose described in FIG. 7. For example, holographic recording materialscan be similarly substituted among the processes.

Although FIGS. 7-9 illustrate specific methods for forming gratings in awaveguide display, many different processes can be implemented to formsuch gratings as appropriate depending on the specific requirements of agiven application. Furthermore, various modifications can be made to themethods shown in FIGS. 7-9. For example, the transmission and reflectiongratings can be formed as multiplexed gratings. In some embodiments,multiplexed transmission and reflection gratings can be formed byinterfering the zero order and diffracted light and interfering thereflection of the diffracted light from the partially reflective surfacewith the zero-order light. In other embodiments, the reflection of thezero-order light and the diffracted light are interfered to form thegrating. The transmission and reflection gratings can also be formedacross different grating layers.

Waveguides implementing transmission and reflection gratings inaccordance with various embodiments of the invention can be implementedwith a variety of grating configurations. In many embodiments, thewaveguide supports at least one input transmission grating, at least onefold transmission grating, and at least one output transmission grating.At least one of the input, fold, and output transmission gratings can bemultiplexed with a reflection grating. In other embodiments, thereflection grating overlaps at least one of the input and fold gratings.In some embodiments, the waveguide supports first and second foldtransmission gratings. The first and the second fold transmissiongratings can overlap each other and at least one reflection grating. Ina number of embodiments, the first and second fold transmission gratingshave crossed K-vectors. Each of the fold transmission gratings can beconfigured to beam-expand light from the input grating in a firstdirection and couple it towards the other fold transmission grating,which can then beam-expand the light in a different direction andextract it towards a viewer.

FIG. 10 conceptually illustrates a waveguide display architectureimplementing multiplexed transmission and reflection fold gratings inaccordance with an embodiment of the invention. In the illustrativeembodiment, the waveguide display 1000 includes a waveguide 1001supporting an input grating 1002, multiplexed transmission 1003 andreflection 1004 gratings, and an output grating 1005. Optical paths forinput light 1006, waveguided light 1007, first direction beam-expandedlight 1008, and second direction beam-expanded output light 1009 areillustrated. In some embodiments, the grating structures are configuredto input and output the light on the same side. As can be readilyappreciated, additional embodiments of the invention can include variousgrating configurations. For example, FIG. 11 conceptually illustrates awaveguide architecture 1100 in which the output grating includesmultiplexed transmission 1101 and reflection 1102 gratings while FIG. 12conceptually illustrates a waveguide architecture 1200 in which theinput grating includes multiplexed transmission 1201 and reflection 1202gratings. FIG. 13 conceptually illustrates a waveguide architecture 1300in which there are provided two separate input gratings 1301, 1302 inaccordance with an embodiment of the invention.

Some key problems in conventional waveguide architectures based oninput, fold, and output gratings can be addressed by combining thefunctions of the fold and output gratings. In many embodiments, thedisplay includes a waveguide supporting an input grating and twooverlapping gratings that perform the dual function of expansion andextraction, with each of the overlapped gratings performing eithervertical expansion or horizontal expansion according to the field ofportion being propagated through waveguide. The grating vectors of theinput and overlapped gratings can be arranged in either equilateral orsymmetrical configurations to provide substantially zero resultantvector. FIG. 14 conceptually illustrates a waveguide architecture 1400that includes a waveguide 1401 supporting an input grating 1402 and agrating structure that includes overlapping multiplexed transmissiongratings 1403, 1404 and reflection gratings 1405, 1406 in accordancewith an embodiment of the invention. In many embodiments, each set ofmultiplexed gratings are disposed in a different grating layer. In someembodiments, the two sets are further multiplexed with one another,forming four multiplexed gratings. In the embodiment of FIG. 14, the twoof the gratings can be configured as crossed fold gratings (i.e., foldgratings with K-vectors in different directions) to provide beamexpansion by changing the direction of the guided beam in the plane ofthe waveguide. In a more general sense, the crossed fold gratings canperform two-dimensional beam expansion and extraction of light from thewaveguide. In some embodiments, the transmission gratings are configuredas crossed fold gratings. In a number of embodiments, two sets ofcrossed fold gratings are implemented. The transmission gratings canform one set while the reflection gratings form the second set. In otherembodiment, one of each of the transmission gratings and reflectiongratings form a set of crossed fold gratings. The field-of-view coupledinto the waveguide can include first and second portions. In manyembodiments, the first and second field-of-view portions correspond topositive and negative angles vertically or horizontally. In someembodiments, the first and second portions may overlap in angle space.In several embodiments, the first portion of the field-of-view isexpanded vertically by the first fold and, in a parallel operation,expanded horizontally and extracted by the second fold.

Although FIG. 14 illustrate a specific grating configurationimplementing crossed fold gratings, various other architectures can beutilized as appropriate depending on the requirements of a givenapplication. For example, various different input grating configurationscan be utilized. In some embodiments, a prism is utilized instead of aninput grating. Other input configurations are shown in FIGS. 15 and 16.FIG. 15 conceptually illustrates a waveguide architecture 1500 usingcrossed fold gratings in which the input coupler multiplexestransmission 1501 and reflection 1502 gratings in accordance with anembodiment of the invention. FIG. 16 conceptually illustrates awaveguide architecture 1600 using crossed fold gratings in which twoseparate input coupling gratings 1601, 1602 are provided in accordancewith an embodiment of the invention. As can readily be appreciated, itshould be apparent that many other combinations of multiplexedtransmission and reflection gratings could be used in a waveguidedisplay that provides two-dimensional beam expansion. In someembodiments, one of the reflection gratings can be omitted from the setof multiplexed gratings used in the crossed grating structure.

In many embodiments, the apparatus includes a waveguide in which inputlight is split into two wavelength bands, which follow bifurcated pathseach with a dedicated fold grating. Light can be extracted using a pairof overlapping output gratings with one grating allocated to eachwavelength band. The output gratings can have gratings vectors at 90deg. to each other. The gratings can use of surface relief orholographic type. In many embodiments, the apparatus includes awaveguide supporting overlapping diffractive elements with gratingvectors aligned in the same direction for performing horizontalexpansion and extraction. The gratings can sandwich an electro activematerial enabling switching between clear and diffracting states. Withregard to crossed grating waveguide architectures, the presentdisclosure can incorporate the embodiments and teachings disclosed inU.S. patent application Ser. No. 16/709,517 entitled “Methods AndApparatuses For Providing A Single Grating Layer Color HolographicWaveguide Display” and U.S. patent application Ser. No. 14/620,969entitled “Waveguide Grating Device,” the disclosures of which areincorporated herein by reference in their entireties for all purposes.

The prescriptions and material properties can be determined by reverseray tracing from the eye box to the image source. The grating layer canbe supported by a transparent substrate. The substrates can be a highindex material, optical glass or plastic. In some embodiments, thesubstrate is curved. The grating can be covered by a second substrate,the first and second substrates forming a light guiding structure. Thegrating can be divided into separate grating elements each havedifferent material and grating properties. At least some of gratingelements can be electrically switchable. The gratings can be formed in aholographic photopolymer, a HPDLC material system, uniform modulationHPDLC material system, or any other material systems that includes atleast one LC and one polymer component. The material or gratingproperties can vary in step change or may vary continuously. Themultiplexed transmission and reflection gratings can have prescriptionsoptimized for the purpose of propagating image light of differentwavelength bands, light of different angular bandwidths, and light ofdifferent polarizations. The gratings can be formed using an inkjetdeposition process.

FIG. 17 conceptually illustrates a profile view of a waveguidearchitecture 1700 in which overlapping transmission 1701 and reflectiongratings 1702, 1703 are provided in accordance with an embodiment of theinvention. In the illustrative embodiment, the architecture 1700includes a transmission grating 1701 sandwich by substrates 1704, 1705,each substrate having an outer surface in contact with one of thereflection gratings 1702, 1703. The outer surface of each reflectiongrating is in contact with a substrate 1706, 1707. As shown, thetransmission 1701 and reflection 1702, 1703 gratings are disposed inseparate layers. As can readily be appreciated, all or any combinationof the gratings can be multiplexed. As described in the sections above,the gratings can be used to provide input coupling, beam expansion,and/or beam extraction. Light propagation in the waveguide isschematically represented by ray 1708. The transmission gratingK-vectors (labelled by symbol K and numeral 1709A) are slanted. Thereflection grating Bragg fringes are substantially unslanted withK-vectors labelled by symbol KR and numeral 1709B. Light extraction fromthe waveguide is represented by the ray 1710.

The reflection holograms can be essentially considered stratified indexsystems. In many embodiments, the outer layers of the reflectiongratings can provide environmental isolation by attenuating the guidedbeam so that total internal reflection occurs mainly before the lighthits the outer surfaces of the waveguide. In some embodiments,aberration can be corrected by building compensation functions into thetransmission grating and reflection grating prescriptions. Reflectiveholographic optical elements (R-HOEs) may also enable curved waveguides.FIG. 18 shows a chart 1800 illustrating the diffraction efficiencyversus incidence angle of a reflection grating 1801 and the diffractionefficiency angular bandwidth of a transmission grating 1802 inaccordance with an embodiment of the invention. Typically, thereflection grating has a diffraction efficiency angular bandwidth ofaround 5-6°. In many embodiments, a reflection grating recorded in HPDLCcan be configured to be polarization selective. In several embodiments,the upper and lower reflection gratings can have symmetricprescriptions.

In some embodiments, the waveguide angular bandwidth can be expanded byusing two reflection gratings disposed above and below the transmissiongrating. FIG. 19 conceptually illustrates a profile view of a waveguidearchitecture 1900 in which overlapping transmission 1901 and reflectiongratings 1902A, 1902B, 1903A, and 1903B are provided in accordance withan embodiment of the invention. Again, although the drawings illustratereflection and transmission gratings disposed in separate layers, all orany combination of the gratings can be multiplexed. In the illustrativeembodiment, the architecture 1900 includes a transmission grating 1901sandwich by substrates 1904, 1905, each substrate having an outersurface in contact with one of the reflection gratings 1902A, 1903A. Theouter surface of the outer reflection grating 1902B, 1903B of each ofthe upper and lower pairs is in contact with a substrate 1906, 1907.Light propagation in the waveguide is schematically represented by ray1908, and extracted light is schematically represented by ray 1909. Thetransmission grating K-vectors (labelled by symbol K and numeral 1910A)are slanted while the reflection grating K-vectors (labelled by symbolK_(R1) and numeral 1910B and symbol K_(R2) and numeral 1910C) aresubstantially unslanted. FIG. 20 is a chart 2000 illustrating thediffraction efficiency versus incidence angle of the reflection grating2001, the effective angular bandwidth resulting from the two reflectiongratings 2002, and the diffraction efficiency angular bandwidth of thetransmission grating 2003.

In some embodiments, the apparatus includes at least one grating withspatially varying pitch. In some embodiments, each grating has a fixed Kvector. In many embodiments, at least one of the gratings is a rolledk-vector grating according to the embodiments and teachings disclosed inthe cited references. Rolling the K-vectors allows the angular bandwidthof the grating to be expanded without the need to increase the waveguidethickness. In some embodiments, a rolled K-vector grating includes awaveguide portion containing discrete grating elements havingdifferently aligned K-vectors. In some embodiments, a rolled K-vectorgrating includes a waveguide portion containing a single grating elementwithin which the K-vectors undergo a smooth monotonic variation indirection. In some of the embodiments, describe rolled K-vector gratingsare used to input light into the waveguide.

In some embodiments directed at displays using unpolarized lightsources, the input gratings used in the invention combine gratingsorientated such that each grating diffracts a particular polarization ofthe incident unpolarized light into a waveguide path. Such embodimentsmay incorporate some of the embodiments and teachings disclosed in thePCT application PCT/GB2017/000040 “Method and Apparatus for Providing aPolarization Selective Holographic Waveguide Device,” the disclosure ofwhich is incorporated herein by reference in its entirety for allpurposes. The output gratings can be configured in a similar fashion sothe light from the waveguide paths is combined and coupled out of thewaveguide as unpolarized light. For example, in some embodiments theinput grating and output grating each combine crossed gratings with peakdiffraction efficiency for orthogonal polarizations states. In someembodiments, the polarization states are S-polarized and P-polarized. Insome embodiments, the polarization states are opposing senses ofcircular polarization. The advantage of gratings recorded in liquidcrystal polymer systems, such as SBGs, in this regard is that owing totheir inherent birefringence they exhibit strong polarizationselectivity. However, other grating technologies that can be configuredto provide unique polarization states may be used.

In some embodiments using gratings recorded in liquid crystal polymermaterial systems at least one polarization control layer overlapping atleast one of the fold gratings, input gratings or output gratings may beprovided for the purposes of compensating for polarization rotation inany the gratings, particularly the fold gratings, which the inventorshave found may result in polarization rotation. In some embodiments, allof the gratings are overlaid by polarization control layers. In someembodiments polarization control layers are applied to the fold gratingsonly or to any other subset of the gratings. The polarization controllayer may include an optical retarder film. In some embodiments based onHPDLC materials, the birefringence of the gratings may be used tocontrol the polarization properties of the waveguide device. The use ofthe birefringence tensor of the HPDLC grating, K-vectors and gratingfootprints as design variables opens up the design space for optimizingthe angular capability and optical efficiency of the waveguide device.In some embodiments, a quarter wave plate disposed on a glass-airinterface of the waveguide rotates polarization of a light ray tomaintain efficient coupling with the gratings. For example, in oneembodiment, the quarter wave plate is a coating that is applied tosubstrate waveguide. In some waveguide display embodiments, applying aquarter wave coating to a substrate of the waveguide may help light raysretain alignment with the intended viewing axis by compensating for skewwaves in the waveguide. In some embodiments, the quarter wave plate maybe provided as multi-layer coating.

The construction and arrangement of the systems and methods as shown inthe various exemplary embodiments are illustrative only. Although only afew embodiments have been described in detail in this disclosure, manymodifications are possible (for example, variations in sizes,dimensions, structures, shapes and proportions of the various elements,values of parameters, mounting arrangements, use of materials, colors,orientations, etc.). For example, the position of elements may bereversed or otherwise varied and the nature or number of discreteelements or positions may be altered or varied. Accordingly, all suchmodifications are intended to be included within the scope of thepresent disclosure. The order or sequence of any process or method stepsmay be varied or re-sequenced according to alternative embodiments.Other substitutions, modifications, changes, and omissions may be madein the design, operating conditions and arrangement of the exemplaryembodiments without departing from the scope of the present disclosure.

DOCTRINE OF EQUIVALENTS

While the above description contains many specific embodiments of theinvention, these should not be construed as limitations on the scope ofthe invention, but rather as an example of one embodiment thereof. It istherefore to be understood that the present invention may be practicedin ways other than specifically described, without departing from thescope and spirit of the present invention. Thus, embodiments of thepresent invention should be considered in all respects as illustrativeand not restrictive. Accordingly, the scope of the invention should bedetermined not by the embodiments illustrated, but by the appendedclaims and their equivalents.

What is claimed is:
 1. A waveguide display, comprising: a source oflight modulated with an image data; and a waveguide comprising: at leastone transmission grating; at least one reflection grating, wherein saidat least one reflection gratin and said at least one transmissiongrating at least partially overlap; and at least one input coupler forcoupling light from said source of light into a total internalreflection (TIR) path in said waveguide.
 2. The waveguide display ofclaim 1, wherein said at least one reflection grating and said at leastone transmission grating are multiplexed in a single grating layer. 3.The waveguide display of claim 1, wherein said at least one inputcoupler is a grating.
 4. The waveguide display of claim 1, wherein: saidat least one input coupler comprises an input transmission grating; saidat least one transmission grating comprises a fold transmission gratingand an output transmission grating; and at least one of said input,fold, and output transmission gratings is multiplexed with said at leastone reflection grating.
 5. The waveguide display of claim 1, wherein:said at least one input coupler comprises an input transmission grating;said at least one transmission grating comprises at least first andsecond fold transmission gratings; said at least one reflection gratingoverlaps at least one of said input transmission grating and said firstand second fold transmission gratings; said first and second foldtransmission gratings overlap each other; said first and second foldtransmission gratings have crossed K-vectors; and each of the foldtransmission gratings is configured to beam-expand light from the inputgrating and couple it towards the other fold transmission grating, whichthen beam-expand and extract light towards a viewer.
 6. The waveguidedisplay of claim 1, wherein each of said gratings has a grating vectorthat in combination provide a resultant vector with substantially zeromagnitude.
 7. The waveguide display of claim 1, wherein said lightundergoes a dual interaction within at least one of said gratings. 8.The waveguide display of claim 1, further comprising a beam splitterlayer overlapping said at least one reflection grating.
 9. The waveguidedisplay of claim 1, further comprising an alignment layer overlappingsaid at least one reflection grating.
 10. The waveguide display of claim1, wherein said source of data modulated light is one selected from thegroup consisting of: a laser-scanning projector, a microdisplay panel,and an emissive display.
 11. The waveguide display of claim 1, whereinsaid source of light provides at least two different wavelengths. 12.The waveguide display of claim 1, wherein at least one of said gratingsis characterized by a spatial variation of a property selected from thegroup consisting of: refractive index modulation, K-vector, gratingvector, grating pitch, and birefringence.
 13. The waveguide display ofclaim 1, wherein said gratings are configured to provide a plurality ofseparate optical paths for a property selected from the group consistingof: wavelength band, angular bandwidth, and polarization state.
 14. Thewaveguide display of claim 1, wherein said waveguide is curved.
 15. Thewaveguide display of claim 1, wherein said waveguide incorporates a GRINstructure.
 16. The waveguide display of claim 1, wherein said waveguideis plastic.
 17. The waveguide display of claim 1, wherein at least oneof said gratings comprises a structure selected from the groupconsisting of: a switchable Bragg grating recorded in a holographicphotopolymer a HPDLC material, a switchable Bragg grating recorded in auniform modulation holographic liquid crystal polymer material, a Bragggrating recorded in a photopolymer material, and a surface reliefgrating.
 18. A method of fabricating a holographic waveguide, the methodcomprising: providing at least one light source, a layer of holographicrecording material, and an at least partially reflective surface;forming first and second recording beams using said at least one lightsource; transmitting said first and second recording beams into saidlayer of holographic recording material; transmitting a portion of saidfirst recording beam through said layer of holographic recordingmaterial towards said at least partially reflective surface; reflectingsaid transmitted portion of said first recording beam off said at leastpartially reflective surface back into said layer of holographicrecording material; forming a transmission grating in said layer ofholographic recording material using said first and second recordingbeams; and forming a reflection grating in said layer of holographicrecording material using said reflected portion of said first recordingbeam and said second recording beam.
 19. The method of claim 18, furthercomprising: forming a liquid crystal and polymer anchoring structure forsupporting a reflection grating.
 20. A method of fabricating aholographic waveguide, the method comprising: providing a mastergrating, a substrate supporting a layer of recording material, a sourceof light, and an at least partially reflective surface disposed oppositeto said master grating with respect to said layer of recording material;illuminating said master grating with light from said source of light toform a diffracted beam and a zero-order beam; reflecting said diffractedbeam from said at least partially reflective surface; forming atransmission grating from said zero-order beam and said diffracted beam;and forming a reflection grating from said zero-order beam and saidreflected diffracted beam.